Serum samples were obtained from patients with RA (n = 155), PD subjects (n = 48), and HCs (n = 35). The profile of antibody response to gingipain RgpA-specific domains, a cysteine protease produced by Pg, was determined in all included patients’ sera, and the potential protective effects of RgpA domains in an experimental arthritis model were also tested. The gene discussed is CTSB; the disease is rheumatoid arthritis.